The invention relates to a device for the ratiometric measurement of sensor signals, particularly in motor vehicle electronics.
Motor vehicle internal combustion engines are increasingly being fitted with an electronic gas pedal (E gas) and electronic traction control (ETC). To determine the position of the gas pedal, throttle valve, air mass flowmeter, etc., potentiometers with a reference voltage applied to them, for example, are used as pedal value transmitters, throttle valve transmitters, air mass transmitters, etc.; the ratios of the voltages which can be measured at their taps to the reference voltage are a measure of the respective position of the associated transmitter.
DE 196 29 747 A1 discloses a sensor device which has a first reference voltage applied to it and generates a ratiometric measured signal; the ratio of the first to a second reference voltage is formed and is used to correct the measured signal.
For reasons of operational reliability, such transmitters are usually-embodied in multiples and are supplied from mutually independent voltage sources.
The demands on the measurement accuracy of sensors in motor vehicle electronics are becoming higher and higher. Many measured values are transmitted from the sensors to a processing microcontroller xcexcC as analog DC voltages Vmexcex2 (0 . . . 5V+0 . . . 100%). The microcontroller digitizes them using an analog/digital converter ADC (e.g. 10 bit ADC: 0 . . . 5V+0 . . . 1023 steps). In an analog/digital converter, the measured voltage Vmexcex2 to be converted is set in relation to a reference voltage. To convert the analog values to digital values as accurately as possible, it is therefore desirable for the sensors and the analog/digital converter ADC to use the same reference voltage.
In motor vehicle electronics, this is generally not possible because the reference voltage used for the analog/digital converter ADC integrated in the microcontroller xcexcC is the supply voltage V5int for the microcontroller xcexcC (VAref=V5int). Since, however, the microcontroller and hence the engine/transmission control unit need to remain operational in the event of a short circuit, for example between a sensor line and ground or the battery, the supply voltage V5int for the microcontroller xcexcC must not be routed to the vehicle wiring harness. For this reason, mutually independent supply and reference voltages are required.
A principal cause of measurement errors is the synchronism of the individual supply and reference voltages. For an engine control unit ST containing a microcontroller xcexcC, the following voltages are required, for example, in the case of dual transmitters:
an internal supply voltage V5int for the microcontroller xcexcC, which is simultaneously the reference voltage for an analog/digital converter ADC integrated in said microcontroller and required for ratiometric measurements of the transmitter voltages and the conversion thereof to digital values required for further processing,
a reference voltage V5a for the first transmitter set SS1 (pedal value transmitter 1, throttle valve transmitter 1, air mass transmitter 1, etc.), and
a reference voltage V5b for the second transmitter set SS2 (pedal value transmitter 2, throttle valve transmitter 2, air mass transmitter 2, etc.). In this context, the notation V5int signifies the internal 5 volt supply voltage for the microcontroller xcexcC, whereas V5a and V5b signify the 5 volt reference voltages for sensors arranged outside of the engine control unit ST. The supply voltage for microcontrollers and sensors is usually 5V.
As already mentioned, the voltages V5int, V5a and V5b need to be independent of one another, i.e. the other voltages must not be impaired in the event of one of these voltages being shorted.
In addition, the supply voltage V5int and hence the operating range of the microcontroller xcexcC need to be monitored. If the supply voltage is outside the prescribed operating range of, for example, V5int 5Vxc2x1250 mV, reliable operation of the engine controller is no longer ensured. Subsequently possible uncontrolled operation of the microcontroller xcexcC and hence of the engine control unit ST can result in personal injury and should be avoided absolutely. The microcontroller xcexcC needs to be stopped in this case.
optimum synchronism would be possible by integrating all the voltage sources in an integrated voltage controller having a common reference voltage source; however, this is hampered by the fact that, in the event of a fault in this common reference voltage source (for example as a result of faulty assembly or as a result of interference pulses), reliable voltage monitoring is not possible because such a fault cannot be recognized by the microcontroller xcexcC.
One implemented compromise solution consists, as shown in FIG. 1, in an inherently known, first voltage controller SR1 generating the internal supply voltage V5int and the external reference voltage V5a using a common reference voltage Ref1 which is obtained from an operating voltage UB and is monitored by a monitoring circuit xc3x9cW integrated in the first voltage controller SR1, and in a second, smaller voltage controller SR2 with a dedicated reference voltage source Ref2 generating the external reference voltage V5b. The monitoring circuit xc3x9cW, shown as a box inside of the first voltage controller SR1, has a window comparator which is indicated by two operational amplifier symbols and whose output signal, possibly delayed by means of a timer xcfx84, is output to the microcontroller xcexcC as a reset signal Res1 if the voltage reference exceeds or falls below prescribed limit values.
The timer xcfx84 is intended to permit a reset signal to be output only if the voltage reference exceeds or falls below the limit values for a particular time.
This embodiment produces good synchronism between the internal supply voltage V5int and the external reference voltage V5a, but loses the synchronism with respect to the reference voltage V5b. Reliable voltage monitoring is not possible in the event of a fault in this common voltage reference Ref1 either, because the limit values in the monitoring circuit xc3x9cW are derived from the voltage reference Ref1 and are then likewise erroneous.
It is the object of the invention to provide a voltage supply device which permits good synchronism between all the voltages generated and ensures reliable voltage monitoring for the supply and reference voltages.
The invention achieves this object by means of a device for ratiometric measurement of sensor signals, comprising a voltage controller constructed to generate a first reference voltage from a received operating voltage, generate a supply voltage for a microcontroller, generate a sensor reference voltage for each one of a plurality of sensor elements selected as a sensor or a sensor set, and derive the sensor reference voltage, for each one of the plurality of the sensor elements, from the first reference voltage independently from the supply voltage.
A first monitoring circuit generates a first reset signal for stopping the microcontroller if the first voltage reference leaves a first prescribed voltage range.
A second monitoring circuit is constructed to generate a second reference voltage from the operating voltage independently from the first reference voltage, and generate a second reset signal for stopping the microcontroller if the supply voltage or the sensor reference voltage leaves a second prescribed voltage range.